This invention relates to alkaline cells, and particularly to alkaline galvanic cells having zinc electrodes and alkaline electrolyte, together with a nickel positive electrode. The invention provides low toxicity rechargeable zinc electrodes for such cells.
The performance of rechargeable zinc electrodes in alkaline electrolytes is the subject, for example, of JONES U.S. Pat. No. 4,358,517, issued Nov. 9, 1982. This patent teaches a nickel-zinc cell where the zinc electrode has a copper grid and an active material that comprises zinc-rich particles, calcium-rich particles, and an entanglement of cellulose fibres. Lead compounds may also be added to improve turn around efficiency and to reduce water loss.
The use of buffered electrolytes is also contemplated, for improvement of rechargeable zinc cells. ADLER et al U.S. Pat. No. 5,453,336, issued Sep. 26, 1995, teaches the use of an electrolyte that contains one or more hydroxides of an alkali metal, one or more fluorides of an alkali metal, and one or more carbonates of an alkali metal.
Another patent which teaches a ternary electrolyte for secondary electrochemical cells is CARLSON U.S. Pat. No. 4,273,841, issued Jun. 16, 1981, which teaches an aqueous alkaline solution having potassium hydroxide, potassium fluoride, and potassium phosphate.
EISENBERG U.S. Pat. No. 4,224,391 issued Sep. 23, 1980, and U.S Pat. No. 5,215,836 issued Jun. 1, 1993, each describe electrolyte formulations that employ mixtures of potassium hydroxide and boric, phosphoric, or arsenic acids.
However, it should be noted that the latter patent describes advantages of alkali fluorides in the range of 0.01 moles to 1 mole; and thus, more alkaline solutions may be employed than are taught in the prior Eisenberg patent.
Other patents are specifically directed to the use of additives to the negative zinc electrodes of alkaline zinc batteries and cells. They include the following:
CHARKEY U.S. Pat. No. 5,556,720, issued Sep. 17 1996, teaches the use of a zinc negative electrode that has zinc active material, barium hydroxide, and strontium hydroxide, in a conductive matrix which includes a metallic oxide that is more electropositive than zinc. However, the zinc negative electrode is split into electrode assemblies which are separated by a hydrophobic element.
HIMY et al U.S. Pat. No. 4,084,047 teaches a zinc electrode that has a minor amount of one or more of a number of oxides including tantalum oxide, lead oxide, cadmium oxide, tin oxide, gallium oxide, and indium hydroxide. All of these are intended to reduce shape change, and to inhibit hydrogen gassing.
Another patent which teaches the use of zinc fluoride and zinc titanate is BADYANATHAN U.S. Pat. No. 4,304,828, issued Dec. 8, 1981.
Also, SANDERA et al U.S. Pat. No. 4,017,665, issued Apr. 12, 1977, teach that the direct additions of alkali fluoride to a zinc electrode has been found to be beneficial. This prevents the reversible movement of zinc into and out of solution in the electrolyte, thereby precluding undesired shifting of the active material on the collector electrode and thereby precluding shape change.
As noted, pasted nickel hydroxide electrodes can have their performance improved by the addition of conductive diluents to improve active material utilization, and such as to establish a conductive insoluble CoOOH network in situ within a pasted nickel hydroxide electrode.
However, the problem of low positive electrode efficiency is exacerbated, sometimes significantly, when their use in nickel zinc batteries is considered, because of the electrolyte requirements of the zinc electrode.
Of course, it is well known that performance inhibiting disfigurement or rearrangement of zinc electrodes can occur during the cycling process of rechargeable zinc electrodes in alkaline electrolytes. Such disfigurement can be minimized in more dilute alkali hydroxide solutions.
The Jones Patent, noted above, appears to be somewhat effective in extending cycle life of the cell by the addition of calcium hydroxide to the zinc electrode.
It has also been noted that buffered electrolytes with or without fluoride additions may also result in increased zinc electrode lifespan. They are particularly described in the Adler et al patent, noted above, where a mixture of alkaline electrolyte having a strength of 2M to 12M is combined with a carbonate of 0.5M to 4M, and a fluoride of 0.5M to 4M.
The Carlson Patent, noted above, describes a mixture that employs 5% to 10% of hydroxide, 10% to 20% of phosphate, and 5% to 15% of fluoride.
Of course, a purpose of the present invention is to provide a formulation for a negative electrode that avoids issues of toxicityxe2x80x94thereby avoiding use of such elements as mercury. At the same time, because of the fact that nickel is one of the most expensive components of a nickel zinc cell, it is intended to provide the highest utilization of the nickel electrode as may be possible.
To that end, the present invention provides a nickel zinc alkaline cell having a zinc oxide electrode supported on a conductive substrate, an alkaline electrolyte, and a positive electrode having nickel hydroxide paste supported on a conductive substrate.
The negative zinc oxide electrode comprises 85% to 95% zinc oxide powder, 1% to 10% bismuth oxide, 1% to 2% of a binder, and 0.05% to 5% by weight of a fluoride salt.
The fluoride salt is a salt of a metal which is chosen from the group consisting of: sodium, potassium, rubidium, caesium, lithium, and mixtures thereof.
Typically, the fluoride salt is potassium fluoride, which may be present in the amount of 0.05% to 4.5% by weight of the zinc oxide. 0.5% has been found to be quite successful.
Also, typically, the bismuth oxide is present in the amount of 3% to 9% by weight of the zinc oxide.
Also, it has been found particularly to be effective when the alkaline electrolyte comprises a mixture of sodium hydroxide, potassium hydroxide, lithium hydroxide, and an acid chosen from the group consisting of: boric acid, phosphoric acid, and mixtures thereof.
When boric acid is present, it has a concentration of 0.6 to 1.3 moles per liter.
The stoichiometric excess of alkali hydroxide is between 2.5 moles and 5.0 moles.
Also, lithium hydroxide is present in the amount of about 0.05 to 3.0 moles.